The abovedescribed type of front bicycle derailleur is disclosed in, for example, Japanese Utility Model Publication Gazette No. Sho 43-19,064. This publication discloses a derailleur in which two parallel linkage members are supported swingably through pivot shafts to a fixing member fixed to the bicycle frame, and a chain guide is pivotally supported to the utmost ends of the linkage members through pivot shafts parallel to the aforesaid pivot shafts to thereby form a linkage mechanism. The linkage mechanism is deformed to move the chain guide axially of multistage front gears, to shift thereby a driving chain from one to another of the front gears.
Generally, the pivot shafts are mounted lengthwise of the chain guide, that is, longitudinal of the bicycle. In the above described conventional derailleur, the axis of each pivot shaft is slanted at the longitudinally fore end downwardly with respect to the rear end.
Accordingly, in such construction, the chain guide, when the linkage mechanism is deformed, moves axially of the front gear and simultaneously longitudinally of the bicycle, in comparison with front derailleurs in which the pivot shafts are disposed horizontally, thereby reducing a contact resistance between the chain guide and the driving chain and enabling the bicycle speed to be changed with a light touch. The pivot shafts, which extend longitudinally of the bicycle and parallel to the chain guide, move in a circular arc with respect to the axial direction of the front chain gear.
In order to ensure the shifting of the chain to a desired one of the front gears, the amount of chain guide movement axially of the front chain gear and the amount of movement thereof longitudinally of the bicycle are determined with respect to a stroke required to operate a control wire, so that when the chain is shifted from a smaller diamater chain gear to a larger diameter one, the chain guide moves along a circular arc around the axis longitudinally of the bicycle, in other words, in a circular arc with respect to the axial direction of the front gear. This type of conventional front derailleur will create the following problem.
When the front derailleur is mounted on the bicycle frame, since the front gears are not constant in mounting position with respect to the bicycle frame, a ratio of the axial movement to the longitudinal movement of the chain guide varies greatly when the mounting position thereof changes.
Hence, for example, when the chain guide is mounted to ensure a predetermined axial movement, the amount of longitudinal movement is reduced sacrificing improving the speed control efficiency, and when mounted to ensure a predetermined amount of longitudinal movement to improve the speed control efficiency, the amount of axial movement is sacrificed so as to lower the speed control efficiency.